Testing in software development is done to validate if a product does what it is supposed to do. With that validation you can provide the nescecary information to the stakeholders.
Unit vs Integration Tests
Unit-testing is a way to test the smallest "unit" (Hence the name) of code. It is used to check if a small snippet of code does the right thing with a certain input.
Another type of testing that you see a lot is: "Integration-testing". Integration tests check the whole system, to check if the system does what the user expects it to do.
Using a Plugin